Photoacoustic and luminescence spectra study on the effects of chlorine substituent on the energy transfer of Eu(III)-chlorobenzoic acid

Article abstract of DOI:10.1016/j.saa.2006.02.051

The photoacoustic (PA) amplitude spectra of three complexes of Eu(III) combined with chlorobenzoic acid (Eu(o-ClC₆H₄CO₂)₃·H₂O, Eu(m-ClC₆H₄CO₂)₃·H₂

Full text of DOI:10.1016/j.saa.2006.02.051

Spectrochimica Acta Part A 66 (2007) 273–277
Photoacoustic and luminescence spectra study on the effects of chlorine
substituent on the energy transfer of Eu(III)–chlorobenzoic acid
∗
Bin Hu, Da Chen, Qingde Su
Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China
Received 21 August 2005; accepted 23 February 2006
Abstract
The photoacoustic (PA) amplitude spectra of three complexes of Eu(III) combined with chlorobenzoic acid (Eu(o-ClC
6
H
4
CO
2
)
3
·H
2
O, Eu(m-
ClC CO O and Eu(p-ClC CO O) have been measured, and the PA phase of the different complexes have been calculated. Both
6
H
4
2
)
3
·H
2
6
H
4
2
)
3
·H
2
the PA amplitude spectra and the luminescence spectra reflect the variation of the luminescent properties, and the PA phase is directly relative to
the relaxation time. Since the relaxation is the process of the intramolecular energy transfer between the ligands and the central ion, the molecular
structure of ligand is the important factor to decide the energy gap between the lowest triplet state of ligand and the resonance level of central ion.
The effects of chlorine substituent on the molecular structure and energy gap of the complexes have been studied by PA phase and luminescence
spectra.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Photoacoustic phase; Chlorine substituent; Intramolecular energy transfer
1
. Introduction
can be a direct monitor of energy gaps, non-radiative relaxation
processes and the complement of fluorescence spectroscopy [7].
One of the important characteristics of PAS is that it provides
information about both amplitude and phase of response of a
sample [8]. The PA amplitude signal depends on a combination
of the optical absorption properties and the thermal properties
of the sample. The phase of the PA signal in response to the
incident light is due to a time delay, which is caused by the ther-
mal response of the sample and by the non-radiative relaxation
process [9,10].
The great interest in the study of lanthanide complexes is
mainly due to their potential practical applications such as laser
materials, luminescent labels in the fluorescence analysis and
biology system [1–3]. A study of the intramolecular relaxation
processes of lanthanide complexes will provide much informa-
tion on the luminescent properties, the situation of the energy
levels and the energy transfer process. If the absorption of excita-
tion energy takes place in ligand instead of central ion, excellent
luminescent properties of lanthanide complexes are attributed to
the intramolecular energy transfer between the ligands and the
central ion [4].
Photoacoustic spectroscopy (PAS) is a recently developed
technique. ThePAresponseisgeneratedwhenopticalabsorption
causes a rapid local heating in the sample under investigation.
The subsequent periodic expansion and contraction of the gas in
theairtightPAcellgeneratesapressurewavethatisdetectedwith
a microphone [5]. PAS is being used extensively in the optical
and thermal characterization of many kinds of materials [6]. It
Lanthanide complexes with aromatic carboxylic acids are
frequently used as structural and functional probes in biolog-
ical systems [11]. PA technique is suitable to the study the
intramolecular energy transfer properties of lanthanide com-
plexes. The intramolecular transfer of energy is related with
the triplet state of ligand (donor) and the excited state of cen-
tral ion (acceptor) [12]. Therefore the suitability of the energy
gap between the lowest triplet state of ligand and the reso-
nance level of central ion is a critical factor for efficient energy
transfer. When a substituent is attached to the ring of ligand at
different positions, the molecular structure of ligand and fur-
thermore the energy gap mentioned above will be changed.
In this paper, solid complexes: Eu(o-ClC H CO ) ·H O,
∗
Corresponding author. Tel.: +86 551 3606642; fax: +86 551 3606642.
6
4
2 3
2
E-mail address: qdsu@ustc.edu.cn (Q. Su).
Eu(m-ClC6H4CO2)3·H2O and Eu(p-ClC6H4CO2)3·H2O are
1
386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.02.051

2
74
B. Hu et al. / Spectrochimica Acta Part A 66 (2007) 273–277
synthesized. Their PA and luminescence spectra are determined
and interpreted.
2
. Experimental
2
.1. Preparation of complexes
Benzoic acid can combine rare earth ions to form stable
complexes. Solid complexes of Eu(ClC H4CO2)3·H2O were
6
synthesized according to the procedure given in Ref. [13]. The
rare earth oxide was converted to chloride by treatment with con-
centrated HCl. A solution of the chloride in absolute ethanol was
added to a solution of ligand in the same solvent. The precipita-
tion was accomplished by slow addition of aqueous ammonia.
The complexes were recrystallized, washed with a little ethanol
and the solution was filtered. Products were obtained after dried
at room temperature and then stored in desiccator.
Fig. 1. PA amplitude spectra of Eu(III) complexes at a chopping frequency of
The results of the elemental analyses and infrared spectra
were consistent with their molecular structures.
12 Hz: (1) Eu(p-ClC H CO ) ·H O; (2) Eu(m-ClC H CO ) ·H O; (3) Eu(o-
6
4
2 3
2
6
4
2 3
2
ClC6H4CO2)3·H2O.
be omitted. Under this condition, Eq. (1) is simplified to
2
.2. Measurement of spectroscopy
P = K1Aabs(1 − η)
(2)
The PA spectra were obtained on a single-beam spectrometer
As the PA amplitude intensity (P) of the sample increases, the
luminescence efficiency (η) exhibits a corresponding decrease.
In Fig. 1, the intensities of PA amplitude signals of energy levels
constructed in our laboratory [14]. A 500 W Xenon lamp, a CT-
0F monochromator, a CH-353 chopper and a PA cell fitted with
3
an ERM10 electret microphone were used. The output signal of
the microphone was fed to a lock-in-amplifier (LI-574A) and
was collected on an A/D converter and dealt with by computer.
The chopper frequency was set at 12 Hz. The PA spectra were
recorded in the region of 300–800 nm, and the amplitude spectra
were normalized against the carbon black to account for the
variation due to the light source and spectrometer.
3+
of Eu , which have fluorescence properties, are quite weak or
3
+
5
vanished. For Eu , the relaxation of D0 cannot be monitored
5
5
by PA spectroscopy, and the PA signals of D1 and D2 at 469
and 540 nm are also very small [19].
Because of the similar molecular structures, the PA intensities
of three samples at the absorption of ligand approach to each
other, especially for the Eu(o-ClC H4CO2)3·H2O and Eu(m-
6
ClC H4CO2)3·H2O. The relative intensities of these samples at
3
. Results and discussion
6
335 nm are shown in Table 1.
3
.1. Photoacoustic amplitude spectra of the complexes
3.2. Luminescence spectra of the complexes
The PA amplitude spectra of Eu(ClC H4CO2)3·H2O at a fre-
6
Fig. 2 shows the luminescence emission spectra of Eu3+
complexes with chlorobenzoic acid. To study the relaxation pro-
cess, the excitation wavelength was selected at 335 nm, which
matches the PA spectra. The emission peaks at 527, 592 and
quency of 12 Hz are shown in Fig. 1. The broad band located
*
from 300 to 450 nm were assigned to the ␲–␲ transition of
chlorobenzoic acid. For Eu3+ in complexes, only f–f transitions
*
are observed from 300 to 800 nm [15,16]. Since the ␲–␲ tran-
5
7
5
7
6
15 nm are, respectively, assigned to D1 → F1, D0 → F1
sition of ligand is much stronger than the f–f transition [17], the
5
7
3+
3+
and D → F of Eu ion [20,21]. Among these transitions,
f–f transition of Eu locating at 300–450 nm are covered by the
0
7
2
5
*
D → F is the strongest. The relative intensities of three
␲
[
–␲ transition. In general, the PA signal can be expressed as
0
2
samples show the obvious difference and the trend of rising is
contrary to that of PA intensities.
18]:
ꢀ
ꢂ
ꢁ
P = K1Aabs 1 − η −
γi
(1)
3
.3. Photoacoustic phase spectra of the complexes
i
where Aabs is the absorbance of the sample, K1 a coefficient
which is determined by the thermal properties of the sample
and by the spectrometer, η the luminescence efficiency and γi
is the conversion efficiency of the other non-thermal deexcita-
tion channels except the luminescence (such as photochemistry,
photoconductivity, etc.). For the title complexes, there are no
other non-thermal deexcitation process generally, so all γi may
The photoacoustic phase is the time delay that occurs during
the process: from the light absorbed by the sample to the acous-
tic signal being detected by the microphone. The phase data
contain contributions from a number of sources: the geometry
of the photoacoustic cell, the response of the detecting system,
the optical absorption coefficient, the non-radiative deexcitation
paths, etc. In an actual photoacoustic measurement, many of

B. Hu et al. / Spectrochimica Acta Part A 66 (2007) 273–277
275
Table 1
PA and luminescence relative intensity and PA phase data of complexes
◦
PA relative intensity (P) (335 nm)
Luminescence relative intensity
PA phase data ( )
Eu(p-ClC6H4CO2)3·H2O
Eu(m-ClC6H4CO2)3·H2O
Eu(o-ClC6H4CO2)3·H2O
1.00
1.17
1.20
1.00
0.57
0.22
111.8
117.7
121.5
the non-sample related parameters may be maintained constant
and the phase may therefore be used to provide the information
about the non-radiative deexcitation processes. The phase may
be expressed as [22].
Fig. 3. The phase angle of maximum amplitude is sample’s PA
phase and the data list in Table 1.
The PA phase of the Eu(p-ClC H4CO2)3·H2O is the smallest,
6
◦
which is 5.9 smaller than that of Eu(m-ClC H4CO2)3·H2O and
6
◦
ꢃ
ꢄ
9.7 smaller than that of Eu(o-ClC H CO ) ·H O at 335 nm.
6
4
2 3
2
1
+ 2
ψ = tan−
1
+ tan−1(ωτ)
(3)
According to Eq. (4), the bigger the PA phase is, the longer the
relaxation time is.
βμs
where ω = 2πf, f is the modulation frequency, and β, μs,
τ are optical absorption coefficient, thermal diffusion length
3.4. Energy transfer and relaxation processes
(
μs = 1/α, α is the heat diffusion coefficient) and the relaxation
3
.4.1. Models of intramolecular energy transfer and
time, respectively. For Eu(III) complexes with the conjugated ␲
electron ligand, the absorption coefficient is high enough, then
the phase data could be only related with the non-radiative deex-
citation processes [23]:
relaxation processes
The variation of Eu(III) complexes in photophysical proper-
ties (PA amplitude, luminescence and PA phase) are due to the
difference of intramolecular energy transfer.
According to Dexter’s theory [25], the energy transfer rate
constant Psa is given by:
ψ = ψ0 + tan−1(ωτ)
(4)
The phase shifts associated with different complexes are best
observed by the plotting the amplitude versus the phase angle at
the absorption peaks of the ligand [24]. Such plots are generated
by measuring the in-phase signals (I0) and quadrature (I90) sig-
nals simultaneously under selected modulation frequency and
calculating the signal Iφ at detecting angle from
ꢃ
ꢄ ꢅ
2πZ2
Psa =
Fs(E)ξa(E) dE
(6)
h
where Fs(E) represents the observed shape of the emission band
of the triplet state of ligand (donor); ξa(E) is the shape of the
absorption band of the excited state of central ion (acceptor).
Therefore, the suitability of the energy gap between the lowest
triplet state of ligand and the resonance level of Eu is a critical
factor for efficient energy transfer.
In the luminescence experiment the excitation wavelength
was selected at 335 nm, where the ligands have the strongest
absorption. It means that the emission spectra bands of Eu are
Iφ = I0 cosφ + I90 sinφ
Repeat this process by scanning the different values of φ from
(5)
3
+
◦
0
to 180 and then acquire a plot of amplitude versus φ. The PA
phase spectra for different complexes at 335 nm are shown in
3
+
Fig. 2. Luminescence emission spectra of the complexes of: (1) Eu(p-
ClC6H4CO2)3·H2O; (2) Eu(m-ClC6H4CO2)3·H2O; (3) Eu(o-ClC6H4CO2)3·
H2O.
Fig. 3. The variation of the amplitude as a function of phase angle at
335 nm: (1) Eu(p-ClC6H4CO2)3·H2O; (2) Eu(m-ClC6H4CO2)3·H2O; (3) Eu(o-
ClC6H4CO2)3·H2O.

2
76
B. Hu et al. / Spectrochimica Acta Part A 66 (2007) 273–277
acquired from the intramolecular energy transfer from donor to
acceptor instead of the excitation–deexcitation of Eu3+ itself.
The effects of different chlorine substituent on the energy lev-
els of the lowest triplet state of ligands are the points to be
studied.
3.4.2. Effects of chlorine substituent on the energy transfer
and relaxation processes
The PA phase represents the lifetime of triplet state of lig-
and. The smaller the phase is, the shorter the lifetime of triplet
state of ligand should be. From the models in Fig. 4, the energy
5
3+
−1
−1
The resonance level ( D0) of Eu ion is at 17,300 cm [26],
gap (7500 cm ) between the lowest triplet state of benzoic acid
−
1
3+
and the lowest triplet state (T) of benzoic acid is at 24,800 cm
[
and excited state of Eu is not most suitable for intramolecu-
−1
27]. The suitability of energy gap (7500 cm ) can make the
lar transfer because it is rather big. The p-chlorine substitution
energy transfer from ligand to central ion. But ring substitutent
can lead to a change of the energetic disposition of the low-
est triplet states [28]. The inductive effects and the conjugative
effects of ring substituent react on the energy levels simultane-
ously. For that chlorine is a kind of strong electron donor, the
makes the energy gap much smaller than before, so the energy
of Eu(p-ClC H4CO2)3·H2O may transfer from the lowest triplet
6
state of ligand to the excited state of central ion much more
effective and the lifetime of the triplet state becomes shorter.
While the o-chlorine substitution makes the process more diffi-
cult. More energy in this complex may transfer from triplet state
to ground state directly instead of to the central ion. The triplet
*
inductive effects would increase the ␲–␲ energy. The conjuga-
tiveeffectsexceedtheinductiveeffects. Sothelowesttripletstate
of Cl-substituted benzoic acid may lie at a lower energy level
than that of unsubstituted compound [29]. In the research about
the effects of various ring substituents on the phosphorescence
of benzene derivatives, it was reported by Wagner et al. that the
triplet state energies are strongly influenced by substituent posi-
tion [30]. When an electron-donating group, such as chlorine,
is attached to the ring of benzoic acid, carbonyl group and the
substituent reinforce each other in stabilizing the energy state.
However, the effects of the two groups are additive only for 1,4
substitution. So the effect of stabilization from p-substitution is
remarkable and that from m-substitution is only slight. On the
other hand, it was also concluded that the slight destabilizations
caused by o-chlorine should be attributed to some steric hin-
drance, which makes the energy level a little higher than that of
unsubstituted complex.
state of Eu(o-ClC H4CO2)3·H2O is metastable and its lifetime
6
is longer than that of the any other complexes. These inferences
also explain why the PA amplitude of Eu(o-C1C H4CO2)3·H2O
6
at 335 is the strongest and the luminescence intensity of cen-
tral ion is the smallest. For Eu(m-ClC H4CO2)3·H2O, because
6
the decrease of triplet state level attributed to m-substitution is
much smaller than p-substitution, the PA phase, the PA ampli-
tude of ligand and the luminescence intensity of central ion of
Eu(m-ClC H4CO2)3·H2O are between those of the other two
6
samples. All of these inferences have been proved by the exper-
imental results mentioned in Figs. 1 and 2 and Table 1.
From the results above, the models of intramolecular energy
transfer processes of Eu(ClC H4CO2)3·H2O are established.
6
The effects of chlorine substituent on the energy transfer are
proved and explained by PA phase and luminescence spectra.
From the above explanation, a general model of intramolec-
ular energy transfer and relaxation processes of three
4. Conclusions
Eu(ClC H4CO2)3·H2O are showed in Fig. 4. The energy lev-
6
The PA amplitude and luminescence spectra of three
els of lowest triplet states of Eu(p-ClC H4CO2)3·H2O and
6
Eu(ClC H4CO2)3·H2O with different chlorine substituents are
Eu(m-ClC H4CO2)3·H2O are below that of benzoic acid, while
6
6
−
1
measured and the PA phase shifts of the different complexes
are calculated. The conjugative effects of chlorine substituent
and the reinforcing effects between carbonyl group and sub-
stituent on benzene ring make the triplet state energy level of
each sample different from that of unsubstituted benzoic acid.
Because of the best suitability of the energy gap between the
that of Eu(o-ClC H4CO2)3·H2O is above 24,800 cm . Espe-
6
cially, the effect of p-substituent on energy level is the most
remarkable.
3
+
lowest triplet state of ligand and the resonance level of Eu ,
the intramolecular energy transfer in Eu(p-ClC H4CO2)3·H2O
6
can proceed effectively. After the investigations of both photoa-
coustic and luminescence spectra, the models of intramolecular
energy transfer processes are established and applied to explain
the effects of chlorine substituent on the energy transfer.
Acknowledgements
We thank the National Nature Science Foundation of
the People’s Republic of China for supporting this program
(No.20175024).
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Fig. 4. Models of intramolecular energy transfer processes of Eu(p-
ClC6H4CO2)3·H2O, Eu(o-ClC6H4CO2)3·H2O and Eu(m-ClC6H4CO2)3·H2O.
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